@EnterpriseArchitect Apologies for the delayed response, just wanted to check does the issue still persists?
Were you able to delete the MDI groups and re-create the instance?
Check the required permissions/pre-reqs required to create MDI instance
https://learn.microsoft.com/en-us/defender-for-identity/deploy/deploy-defender-identity
Let me know if you have any questions, feel free to post back.